In medicine, it is often necessary to detect changes in muscle thickness during contraction of a muscle which overlies a bone structure. The muscle thickness during contraction is a measure of muscle strength and can indicate the efficacy of neuromuscular blocking drugs.
Devices exist which, when temporarily fixed to the skin above the muscle, non-invasively detect changes in muscle thickness using a variety of techniques. For example, accelerometers can be used to detect the acceleration of the muscle as it contracts. Microphones can be used to detect pressure waves created by contraction of the muscle. These measurements can be then related to changes in muscle thickness. These devices can be effective for large muscles (ex. quadricep muscles).
The accuracy of existing devices in detecting changes in muscle thickness decreases when smaller muscles are monitored. With smaller muscles (ex. muscles in the face or hands), accelerations are small and thus the acceleration of such muscles can be damped considerably by the overlying structures of skin and fat. In addition, such tiny accelerations are easily lost due to large acceleration signals caused by the movement of other muscles in the area or by external acceleration forces such as whole body movement, vibration of the floor, or even pulsatile movement caused by blood flow.
Microphones are more accurate than accelerometers when used for detection of small muscle movement. However, microphones usually require an air or liquid coupling to the body to ensure adequate signal strength. Maintaining an appropriate gap between the active structure of the microphone and the skin, as well as maintaining an adequate and repeatable air-seal is very difficult. Microphones can be made to work in this application, but the attachment becomes problematic and the signal output from such devices tends to change over time due to the sensitivity of the coupling media. Microphones also tend to have a rigid structure, thus the curvature of the contracting small muscle creates a large gap between the muscle and the flat surface of the rigid device, not seen with larger muscles. With smaller muscles, the microphone gives less accurate results during muscle contraction because the microphone simply rides on top of the muscle. A further disadvantage of microphones is their inability to recognize local movements and distant movements. Since bodily fluids and bone are such excellent conductors of sound and vibration, microphones will pickup a combination of local movement and these other, unwanted, vibrational and pressure signals. A final disadvantage of microphones is that they are fundamentally based on sound which has an oscillatory nature. Microphones are therefore unsuited to differentiate between a muscle that is contracting and a muscle that is relaxing as they will both have similar sounds.
When considering the case where muscle overlies a bone structure, further complications exist. The bones themselves often move as the result of activity of several different muscle structures. This is particularly troubling for accelerometers as they cannot differentiate the gross movement of a bone lifting a passive muscle from the movement of the muscle itself actively changing position or shape.
For muscles that overly a flat bone structure, the skin and muscle will often slide over the bone due to unwanted forces. For example, activation of muscles in the cheek will cause detectable movement in the forehead. Accelerometers can account for such movement by employing acceleration detection in multiple axes to differentiate accelerations that are perpendicular to the bone from ones that are tangent to the bone. However, microphone-based systems will not only pickup the vibrations, pressure and sound of the muscle sliding over the bone, but due to the excellent vibration conduction properties of bone, the microphone may falsely interpret such signals as being generated by the muscle being monitored.
There exists a need for a device that will accurately detect changes in muscle thickness during contraction of a small muscle overlying a bone structure.
There further exists a need for a device that can determine if a muscle is contracting or relaxing.